An Estimate of Thorium 234 Partition Coefficients Through Global Inverse Modeling

Abstract

Thorium‐234 (234Th), an insoluble radioisotope scavenged by marine particles, can be used as a proxy of the biological carbon pump. Thorium‐234 observations can constrain biogeochemical models, but a necessary first step is to estimate the poorly known partition coefficients between particulate and dissolved phases. In this study, the 234Th partition coefficients for five particle types, differing in size and chemical composition, are estimated by fitting a global 3‐D 234Th model based on the coupled ocean general circulation‐biogeochemistry model NEMO‐PISCES (at a resolution of 2°) to a global 234Th data set (including GEOTRACES data). Surface partition coefficients are estimated between 0.79 and 16.7×106. Biogenic silica has the smallest partition coefficients. Small particulate organic carbon and lithogenic dust have the largest. Thorium‐234 observations at depth cannot be recovered without allowing partition coefficients to increase by one order of magnitude from surface to 1,000 m deep. In our time‐dependent global 3‐D model, the biases introduced by three common assumptions made in biological carbon pump studies can be quantified. First, using the C:234Th ratio of large particles alone leads to an overestimation of carbon export at the base of the euphotic layer, by up to a factor 2. Furthermore, assuming steady state and neglecting transport by advection and diffusion can bias fluxes by as much as 50%, especially at high latitudes and in upwellings, with a sign and intensity depending on the season.

Revisiting five decades of <sup>234</sup>Th data: a comprehensive global oceanic compilation

“…Previous efforts compiling 234 Th-based data have been created to access 234 Th-derived POC fluxes (see Le Moigne et al, 2013a), total 234 Th activity from the surface to 1000 m depth (Le Gland et al, 2019), and, more recently, POC: 234 Th ratios (see Puigcorbé et al, 2020, and https://doi.org/10.1594/PANGAEA.911424; Puigcorbé, 2019). In contrast, we have compiled the complete results of 234 Th measurements in seawater and particles at every depth, location, and time of sampling.…”

New estimate of organic carbon export from optical measurements reveals the role of particle size distribution and export depth

“…In the field, sediment traps and thorium-234 measurements have been used to quantify sinking particle fluxes. However, both types of observations lack detailed particle size information, vertical resolution, and have known biases, making extrapolations to global scales difficult (K. Buesseler et al, 2007;Le Gland et al, 2019). Recently, optical methods have gained traction to estimate particle export.…”

Constraining the ocean's biological pump with in situ 1 optical observations and supervised learning. Part 2: 2 Carbon Flux

“…In the field, sediment traps and thorium deficit measurements have been used to quantify sinking particle fluxes. However, both types of observations lack substantial global coverage, vertical resolution, and have known biases, making extrapolations to global scale difficult (K. Buesseler et al, 2007;Le Gland et al, 2019). Recently, optical methods have gained traction to estimate particle export.…”

“…These PSD distributions are then combined with empirical relationships that relate particle size and abundance to sinking fluxes (Kriest, 2002;Guidi et al, 2008;Kiko et al, 2017), which we tune against a global data set of in situ sediment trap and thorium-derived particle flux observations (Bisson et al, 2018). Although there are known biases in the trap and thorium flux data (K. Buesseler et al, 2007;Le Gland et al, 2019), they are currently the best estimates of in situ carbon flux. By comparing patterns in particle flux with potential environmental drivers, we gain insight into the mechanisms responsible for particle export and its spatial and temporal variability.…”